1. Field of the Invention:
The present invention relates to the separation of yttrium values from aqueous solutions thereof, and, more especially, to the separation of yttrium values from aqueous solutions thereof including, in addition to said yttrium values, other rare earths.
By the term "rare earths" are intended the elements of the lanthanide group of the Periodic Table having an atomic number ranging from 57 to 71, inclusive, as well as yttrium which has the atomic number 39.
Also, by the term "ceric rare earths" are intended the lightest rare earth elements, beginning with lanthanum and extending to neodymium, and by the term "yttric rare earths" are intended the heaviest rare earth elements, beginning with samarium and concluding with lutetium and also including yttrium.
2. Description of the Prior Art:
It is known to this art that the separation of the rare earths contained in solutions resulting from the digestion of various ores, such as, for example, gadolinite, cerite, monazite, xenotime, bastnasite, and the like, is difficult to carry out, due to the fact that the differences in the properties of a rare earth with respect to those of neighboring elements in the Periodic Table are extremely small.
This separation is even more difficult because the rare earth must be recovered in isolated form with respect to the elements of the same subgroup, such as, for example, the separation of cerium from the other rare earths belonging to the group of "ceric rare earths".
These separations are typically carried out employing extraction processes using an extractant contained in a phase which is not miscible with the generally aqueous solution containing the rare earths.
The processes which are usually employed are liquid/liquid extraction techniques in which the extractant is dissolved in a water-immiscible organic diluent, and techniques entailing the use of resins on which the extractant is adsorbed.
Selective separation of an element with respect to other elements is achieved by judicious selection of the extractant and the extraction conditions. However, it is difficult to predict whether an extractant which is suitable for extracting a given element will be equally suitable for extracting another element. Moreover, the extraction conditions for a particular element also cannot be deduced from the conditions used for the extraction of another element.
Of the rare earths, yttrium is an element which is especially useful for luminophore compositions, as well as for applications in cathodic television tubes and in incandescent lamps.
Thus, a serious need continues to exist in this art for a simple improved process to directly separate yttrium, in a single step, from the other rare earths contained in a solution thereof, especially when yttrium is the predominant such element.
One process for the selective separation of yttrium values is described in U.S. Pat. No. 3,575,687. This process entails using, as the extractant, a mixture of an anionic extractant of the quaternary ammonium compound or phosphonium compound type with a cationic extractant selected from among aliphatic and aromatic phosphates and carboxylic acids.
However, this mixture of extractants presents several disadvantages, such as, for example, mediocre selectivity between yttrium and the heavy rare earths of the "yttric rare earth" group.
Such extractant mixture also presents other disadvantages, in particular a difficult back-extraction of the rare earths contained in the organic phase.